Move changelog to separate file

[fenhl]

The readme has been getting quite long, so this PR moves the changelog to a separate CHANGELOG.md file.

I also took the opportunity to turn the link to ootrandomizer.com into a proper Markdown link.

[flagrama]

Perhaps this is an opportunity to only use the wiki for released changes and only keep dev changes documented in the repo? It would probably also be nice to have a link added to the "full" readme on the wiki which in addition to the content that stays pretty static in the repo readme is regularly updated to indicate what options do and which versions they were added/modified.

[fenhl]

We should keep a full changelog somewhere in the git tree imo. As for the link to the wiki article, I'm not sure it makes sense to link to a document that starts with a copy of the same text you just read. Maybe if the setting release history was a separate document?

[flagrama]

You can always link to headings of wiki pages with as an anchor point. https://wiki.ootrandomizer.com/index.php?title=Readme#Generation_Options